If you want to use the mondate package you will need to specify the day of
the month. Dates "March-2010" and "May-2010" are ambiguous. Recommend you
choose last day of both months as "representative days." Then those days
will be integer months apart.

> a<-mondate("March 31, 2010", displayFormat="%B %d, %Y")
> b<-mondate("May 31, 2010", displayFormat="%B %d, %Y")
> print(a)
[1] March 31, 2010
> print(b)
[1] May 31, 2010
> b-a
[1] 2
attr(,"timeunits")
[1] "months"
> c(b-a)       ## strip away the attribute
[1] 2

Technically speaking, since "mondates" are fundamentally numeric, the result
of b-a is *numeric*, not *integer*, but is as close to the integer 2 as an R
*numeric* can be:
> is.integer(c(b-a))
[1] FALSE
> is.integer(2)
[1] FALSE
> identical(c(b-a),2)
[1] TRUE

Even easier, use the mondate.ymd function, which assumes the last day of the
month if not provided, and you won't have to worry about the number of days
in a month or leap years. You can also retain your "Month-Year" format when
printed if that is a requirement:

> a<-mondate.ymd(2010,3, displayFormat="%B-%Y")
> b<-mondate.ymd(2010,5, displayFormat="%B-%Y")
> print(a)
[1] March-2010
> print(b)
[1] May-2010
> b-a
[1] 2
attr(,"timeunits")
[1] "months"
> identical(c(b-a),2)
[1] TRUE

This works for any "last-day-of-the-month's" because mondate represents them
as numerics with zero fractional part.
Hope that helps,

Dan Murphy
=========================================
Message: 26
Date: Sat, 10 Jul 2010 15:17:07 -0400
From: Gabor Grothendieck <ggrothendieck@gmail.com>
To: Bogaso Christofer <bogaso.christofer@gmail.com>
Cc: r-help@r-project.org
Subject: Re: [R] Need help on date calculation
Message-ID:
       <AANLkTimP7Z4HHWn-qabDMeEIGIYfN0UM_AYmsn5dX_Rt@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

On Sat, Jul 10, 2010 at 3:34 PM, Bogaso Christofer
<bogaso.christofer@gmail.com> wrote:
> Thanks Gabor for your input. However my question is, is your solution
> general for any value of "a" and "b?"
>

#1 and #2 are general.

For #3 I think there is a possibility you might in general have to do
the same tricks as #1 and #2 but am not sure.  Suggest you discuss
with author of mondate package.

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